Electromechanical oscillators are frequently used to measure changes in the physical properties of a given mass, usually by transducing these changes into changes in the operating frequency of the measuring instrument. Examples of such oscillators include density meters, mechanical analyzers, viscometers, moisture meters and temperature meters.
In viscometers, for example, precise amplitude control is a critical requirement. The driving effort needed to maintain oscillation at a constant amplitude, corresponding directly with the rate of decay (or damping) of the amplitude of oscillation, is a function of viscosity of the sample material.
In addition, the elastic properties of the material can be determined by measuring the phase angle between the driving signal and the probe output signal. These relationships are well known; e.g., see U.S. Pat. No. 3,501,952 and R. Darby Viscoelastic Fluids--Marvel Decker (1976).
Referring to the latter reference, it is shown that by maintaining the amplitude of oscillation constant, assuming negligible variation in frequency and a linear sample material, the real or viscous component of viscosity relates directly to the average power required to drive the probe. On the other hand, to determine elasticity (or the complex component of viscosity) it is the small variation in frequency which needs to be measured.
The usual practice, such as disclosed in U.S. Pat. No. 3,712,117 and U.S. Pat. No. 3,710,614, for controlling probe oscillation amplitude has been to rectify (or detect) the amplitude of the oscillator signal, compare it with a d-c setpoint value, and then apply the difference signal as a proportional correction in the strength of the positive feedback for driving the probe. A deficiency in such a control system is that phase delays in detector response to the probe output signal affect loop stability and therefore restrict the amount of proportional gain which the system can tolerate. This limits the degree of precise amplitude regulation which can be applied. Furthermore, in an industrial environment, the inability of a fast response detector system to signal average makes this system particularly sensitive to extraneous background noise.
Phase-locked loop (PLL) circuits have recently been applied in electromechanical systems as a means for automatically tracking rapid changes in a probe frequency with negligible phase error over a narrow band of frequencies and for producing a constant amplitude drive signal. Such an application of the PLL has been described recently by H. M. Simpson and A. Sosin, Automatic Internal Friction and Modulus Measurement Apparatus Utilizing a Phase-Locked Loop--Rev. Sci. Instrum., Vo. 48, No. 11, November, 1977.
Other means for regulating the drive signal to maintain a constant amplitude probe oscillation are known in the art. One such means, described in the foregoing Simpson et al. reference, uses the integrated difference between a d-c setpoint voltage and the d-c converted probe signal to adjust the level of the drive. A problem here is that the system's response to change is rather slow and stability is reduced by the absence of proportional control action. Another means, described in U.S. Pat. No. 4,049,997, switches a drive signal, derived in a manner similar to that described by Simpson et al., at a rate determined by the occurrences of the zero-crossing points of the a-c probe signal. Again, the time response of such systems is somewhat limited.